This invention relates to radio broadcasting and, more particularly, to methods of and apparatus for broadcasting digitally modulated signals and analog amplitude modulated signals within the same frequency channel assignment.
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested, including out-of-band techniques in which the digital radio signals would be broadcast in a specially designated frequency band, and in-band techniques in which the radio frequency signals would be broadcast within vacant slots between adjacent channels in the existing broadcast band (interstitial approach) or within the same frequency channel allocations currently used by commercial broadcasters (in-band on-channel approach). The in-band approach may be implemented without the need for additional frequency coordination and with relatively minor changes to existing transmitting equipment. Of course, any digital audio broadcasting (DAB) technique should not degrade reception by conventional analog receiver circuits.
The use of digital audio broadcasting in the AM band (530 kHz to 1700 kHz) would provide AM broadcasting stations with a means to compete with high quality portable audio sources such as cassette tapes and compact disc players. It would therefore be desirable to extend the in-band on-channel (IBOC) approach to AM broadcasting frequencies to provide enhanced fidelity through digital signaling without affecting reception by existing analog AM receivers.
The broadcasting method of this invention utilizes a composite waveform comprising: an amplitude modulated radio frequency signal, wherein the amplitude modulated radio frequency signal includes a first carrier at a frequency of f0, amplitude modulated by an analog signal representative of program material; and a plurality of digitally modulated orthogonal frequency division multiplexed (OFDM) carrier signals, with the OFDM carrier signals being within a frequency range of about f0xc2x115 kHz, each of the digitally modulated carrier signals being digitally modulated by a portion of a digital signal, wherein the digital signal includes a digital representation of the program material, additional data, or a combination of the program material and the additional data. A first group of the digitally modulated carrier signals overlap the frequency spectrum of the analog amplitude modulated radio frequency signal and are modulated in-quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the frequency spectrum of the analog amplitude modulated radio frequency signal and are modulated both in-phase and in-quadrature with the first carrier signal. A training sequence of known data is periodically transmitted using selected ones of the digitally modulated carrier signals. The program material may be signals representing talk or music as typically broadcast on commercial AM radio stations.
The invention also encompasses a radio frequency transmitter comprising: means for transmitting a composite radio frequency signal, having an amplitude modulated signal including a first carrier at a frequency of f0, amplitude modulated by an analog signal representative of program material, and a plurality of orthogonal frequency division multiplexed (OFDM) carrier signals spaced at about 454 Hz, within a frequency range of about f0xc2x115 kHz, which encompasses the frequency spectrum of the amplitude modulated signal, each of the digitally modulated OFDM carrier signals being digitally modulated by a portion of a digital signal, wherein the digital signal includes a digital representation of the program material, additional data, or a combination of the program material and the additional data. A first group of the digitally modulated carrier signals overlap the frequency spectrum of the analog amplitude modulated signal and are modulated in-quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the frequency spectrum of the analog amplitude modulated signal and are modulated both in-phase and in-quadrature with the first carrier signal.
The invention further encompasses a radio frequency receiver comprising: means for receiving both analog and digital portions of a composite radio frequency waveform, wherein the waveform includes a first signal having a first carrier at a frequency of f0, amplitude modulated by an analog signal representative of program material, and a plurality of orthogonal frequency division multiplexed (OFDM) carrier signals, within a frequency range of about f0xc2x115 kHz, which encompasses the frequency spectrum of the amplitude modulated radio frequency signal, each of the digitally modulated carrier signals being digitally modulated by a portion of a digital signal, wherein the digital signal includes a digital representation of the program material, additional data, or a combination of the program material and the additional data. A first group of the digitally modulated carrier signals overlap the frequency spectrum of the first signal and are modulated in-quadrature with the first carrier. Second and third groups of the digitally modulated carrier signals lie outside of the frequency spectrum of the first signal and are modulated both in-phase and in-quadrature with the first carrier. The analog signal on the first carrier and the digital signal on the digitally modulated carriers are demodulated and an output signal is produced.
This invention provides an in-band on-channel broadcasting method by which digital representations of audio programming material, or other digital data, can be transmitted within an existing AM broadcast channel without adversely affecting existing analog AM receivers and with relatively minor modifications to existing AM transmitting equipment. Transmitters and receivers that transmit and receive signals in accordance with the broadcasting method are also encompassed by this invention.